The present invention relates to an arrangement for attaching machine elements one to another and especially a machine element to a shaft. The invention is particularly suitable for the attachment of impellers of pumps and fans and most preferably it applies to the attachment of impellers of centrifugal pumps.
Several different methods are known for connecting e.g. impellers of centrifugal pumps to their shafts. The most conventional method is to use a flat key as disclosed in, for example, patent publications CH No. 530560 and CH No. 600164. In such a case, the impeller is fitted to the shaft so tightly that a press is needed for mounting the impeller on the shaft and a detaching device for removing it therefrom. Fitting has to be precise because there must be no clearance between the shaft and the impeller or between the key and the keyways which would allow the impeller to move relative to the shaft. If the impeller could move on the shaft, the changes in load during pumping would cause blow-like stresses on the key, which would be easily cut and broken. Furthermore, the problem with the key attachment is that it cannot restrain axial forces which are always generated at the pump impeller. The axial force that draws the impeller outwards from the shaft has to be restrained by a bolt or nut, which secures the attachment of the impeller to the shaft. The securing element, either a bolt or a nut, is subject to continuous stress, which causes a load on its threads, which may result in cutting of the thread.
Another method of attachment is a threaded connection as disclosed in GB patent publication No. 1430308 presenting as an example a turbocharger for the engine exhaust gases, the compressor impeller of which is threadedly fitted as shown in FIG. 2 of the publication. Threaded connections, however, involve several disadvantages which is why some users of centrifugal pumps do not accept this mode of attachment at all. In a threaded connection, the connected parts are normally guided relatively to each other in the side area of the thread. Stresses resulting from both radial and axial guiding are directed to the same area which is also subject, due to a slight pitch, to a great axial stress caused by the impeller torque. Thus, the surface pressures on the sides of the thread increase considerably involving a risk of the thread being cut, which occurrence will still further impede the replacement of the impeller which even now is difficult and time-consuming. Furthermore, if the shaft thread is damaged, the change of impeller becomes an unnecessarily big operation.
The threaded connection does not allow starting of a pump in an abnormal direction of rotation because the thread will open, the impeller moving axially outwards and breaking when hitting the walls of the pump. In such situations, it has been attempted to secure the attachment of the impeller by means of nuts or bolts, but a torque which opens the threaded connection even a small amount causes too great an axial force for a bolt or nut locking to hold. A bolt will break off or the threads of a nut will break. The reason for this is a slight pitch transforming a torque, even if its effect were only a minimal opening, into a high axial force. For these reasons one has to use big thread diameters and big securing elements. In other words, securing may call for stronger structural arrangements than otherwise necessary or possible.
A third method of attachment is conical arrangements as disclosed in CH-PS No. 471974 and CH-PS No. 576074, in the former of which the attachment is secured by a solid nut engagement. The immobility of the shaft and impeller themselves relative to each other is accomplished by the key effect of the cones. In the latter CH publication, the conical attachment is secured by a radial locking nut. Furthermore, heat expansion may be of help at the installation stage. This kind of attaching arrangement with no supporting elements between the impeller and the shaft to prevent their sliding relative to each other is not applicable to centrifugal pumps, at least not in ones with great differences in stock temperatures and with a high torque against stock pumping.
A fourth method of attachment is based on mere heat expansion of materials, but these arrangements have two severe weaknesses. A heat shock caused by some reason or another may suddenly loosen the impeller and make it rotate on its shaft. During maintenance, detaching the impeller of its shaft calls for heating of the impeller as well as plenty of time, just like the installation of a new impeller. Impellers fixed on their shafts by welding also belong to this group. Changing these impellers is at least as difficult as with the previous ones.
A fifth method of attachment is the use of a key shaft in fixing the impeller, as disclosed in DE application No. 2822499. The attachment is similar to the one used in e.g. power transmission equipment of vehicles and equivalent where slight axial sliding allowance is needed. In this application, the axial movement is prevented by a locking nut, which is subject to the whole axial load caused by the suction of the pump. A corresponding structure is also disclosed in GB patent publication No. 1185314, in which the pump shaft is for a part of its length square, said square portion being disposed in the axial center hole of the impeller. The axial securement is accomplished by an end nut.
All present methods of attachment have shortcomings which cause either risks or extra work and call for separate arrangements for preventing dangerous situations. Additional arrangements again normally result in extra time and more equipment needed for the change of a pump impeller.